Background: The in vitro activities of various antibiotics, either alone or in combination with colistin methanesulfonate, were assessed using Pseudomonas aeruginosa strains isolated from cystic fibrosis patients. Methods: Except for colistin methanesulfonate, minimum inhibitory concentrations were determined by microbroth dilution technique as described by the Clinical and Laboratory Standards Institute ( CLSI); for colistin methanesulfonate, a modified method of the CLSI was used. Minimum bactericidal concentrations were determined as described by the CLSI. The in vitro activities of antibiotics in combination were determined by microbroth checkerboard technique, and results were interpreted by fractional inhibitory concentration index. Results: According to MIC values, 100, 98, 96 and 84% of the isolates were found susceptible to amikacin, colistin methanesulfonate, meropenem and ceftazidime, respectively. The minimum bactericidal concentrations were generally equal to or twice as high as those of the minimum inhibitory concentrations. With a fractional inhibitory concentration index of <= 0.5 as borderline, synergistic interactions were more frequent with combinations where amikacin was involved than with those with colistin methanesulfonate. No antagonism was observed. Conclusion: The findings of this study may play a useful role in selecting the appropriate combinations when a single agent is inadequate to treat cystic fibrosis patients with P. aeruginosa infections. Copyright (c) 2008 S. Karger AG, Basel.